Electromechanical rotary actuators are well known and are used in a variety of industrial and consumer applications. They are particularly useful in the field of optical scanning, where an optical element is attached to an actuator output shaft, which is then rotated back and forth in an oscillating manor.
Electromechanical rotary actuators are particularly useful for optical scanning, where an optical element may be attached to an actuator shaft which is rotated in an oscillating manner. In such an application, the actuator and mirror combination redirect a beam of light through a range of angles, or redirect the field of view of a camera so to observe a variety of targets. By way of further example, a prism or an optical filter may be attached to the shaft and the rotation of the actuator shaft varied. If a dielectric filter is used, changing the filter's angle-of-incidence will shift the band-pass wavelength characteristics higher or lower, thus allowing the optical system to be tuned to a particular wavelength. Alternatively, the prism or filter can be rotated completely into and out of the beam path, thus allowing selective filtering of the beam.
Typical electromechanical rotary actuators used for optical scanning are generally made from some combination of magnet, steel and coils of insulated magnet wire. These elements have been arranged in a variety of ways, but for the past twenty years, the most popular arrangement has been to use a simple two-pole rotor magnet, and a toothless stator design.
The rotor within these actuators is typically a solid, cylindrical magnet made from high grade Neodymium Iron Boron which is diametral magnetized, and onto which two shafts are attached. One shaft portion may be attached to a mirror and another shaft portion operable with a position sensor. The shaft is typically supported by ball bearings. By way of example, dimensions for this disclosure may comprise a rotor magnet having a diameter of 0.12 inches (around 3 millimeters) and a length of 1.3 inches (around 33 millimeters).
It will be helpful to review known actuator technology and make reference to known actuators to have the reader better understand the needs satisfied by embodiments of the present invention. While addressing problems in the art in this background section of the disclosure, it will also be helpful to describe developing embodiments generally accomplished through extensive analysis and experimentation. Therefore, all the disclosure included in this background section should not be construed as being a known prior art teaching.
By way of example, FIG. 1 illustrates a cross sectional view of the rotor and stator arrangement found in a typical toothless optical scanner of the current state of the art. The stator is essentially tubular. For the rotor magnet diameter described above, a typical stator tube may have an outside diameter of 0.5 inches (around 12.7 millimeters), an inside diameter of 0.196 inches (around 5 millimeters), and is typically made from cold rolled steel. Coils of magnet wire are formed and bonded to the inside wall of the stator steel tube, occupying around a 90 degree arc. There is typically around a 0.007 inch gap between the outside wall of the rotor magnet and the inside wall of the coil, thus allowing the magnet to rotate freely. Within FIG. 1, the coil areas are designated as Coil plus and Coil minus to indicate turns going into the page and turns coming out of the page, respectively.
FIG. 2 illustrates magnetic field lines found in a typical toothless optical scanner as illustrated in FIG. 1. It can be seen that the magnetic flux lines must extend, jump, across a relatively large gap to reach the stator steel. The coil resides in between the magnet and the stator steel. When the coil is energized, a Lorentz Force is imposed on both the coil and the magnet. Since the coil is typically bonded to the stator and held relatively stationary, all of the force is conveyed to the rotor magnet. Since force is created on opposite sides of the magnet, the force being in the form of torque, the actuator creates torque and thus rotary motion.
In this example of an actuator, there are 50 turns of AWG #33 magnet wire used, having a coil resistance (R) of around 2.5 ohms, and a coil inductance (L) of around 100 microhenries, producing a torque constant (KT) of around 38,000 Dyne*Centimeters torque per Amp of electrical current passing through the coil.
The toothless arrangement provides benefits. One benefit is the relatively low coil inductance that results from the fact that the coil does not completely surround a closed steel core. Quite the contrary, the entire inside of the actuator is open, containing only the rotor magnet whose permeability is almost the same as that of air.
However, the toothless structure is not without drawbacks. One primary drawback is the amount of heat generated during fast/wide angular rotor motions. Further, the heat that is generated cannot be removed effectively. Both of these drawbacks stem from the fact that, the coil occupies a relatively small space (cross-sectional area), and that it is bonded to the inside of the stator tube, so that it only has a direct attachment on one side (the outside of the coil).
Referring again to FIG. 1, it can be seen that the left, right, and inside of the coils are essentially not attached to any surfaces. Because of this, heat generated by the coil can only be removed from one surface (the outside). Indeed, heat generated at the inside surface of the coil tends to heat up the rotor magnet, which degrades performance and can risk demagnetizing the rotor magnet if the heat exceeds around 100 degrees C.
In order to generate less heat, a lower coil resistance is needed, and in order to decrease the coil resistance, thicker wire must be used.
If, for example, AWG #29 magnet wire was used instead of AWG #33 magnet wire, and was placed into the same coil area, only around 22 turns could be used, providing a coil resistance (R) of 0.48 ohms and a torque constant (KT) of 16,720 Dyne*Centimeters per amp. The coil resistance is certainly lower (because of the thicker wire), but the torque constant is also lower (because there are fewer turns).
When comparing motor designs, it is useful to use figures of merit. One important figure of merit is referred to as a motor constant (KM), which indicates the amount of heat generated for a given amount of torque produced by the actuator. The KM can be calculated several ways, but the easiest way is: KM=KT/√R.
The KM of the original actuator with 50 turns, whose KT=38,000 and R=2.5 ohms is 24,033 Dyne*Centimeters per square root of watt. Therefore, to generate 24,033 Dyne*Centimeters of torque, the motor will need to dissipate 1 watt of heat. To generate twice this amount of torque, or 48,066 Dyne*Centimeters, the motor will need to dissipate 4 watts of heat. Doubling the torque output requires doubling the electrical current input. Since heat is proportional to current squared, it illustrates that doubling the current creates four times the heat.
Comparing these values to the same actuator with 22 turns of AWG #29, whose KT=16,720 and R=0.48, reveals that the KM is now 24,133 or, roughly the same as it was before.
This demonstrates an important law of moving magnet actuators. The KM is dictated by the area allocated for the coil. It does not matter how many turns of wire occupy the coil area. If the coil area remains the same and is fully filled with turns, then the KM will remain the same.
For this reason, it is tempting to simply increase the coil area, for example, by increasing the outside diameter of the coil (and inside diameter of the stator tube). However, increasing the diameter of the stator tube will increase the magnetic air-gap, across which the magnetic flux must jump.
Another figure of merit used in magnetic design is referred to as a Permeance Coefficient (PC). The PC indicates an operating point of the rotor magnet. For a simple circuit including a magnet, air, and high permeability steel, the PC may be determined by dividing Magnetic Length by a total magnetic air-gap. For the electromechanical actuator described above with reference to FIG. 1, having a rotor diameter (magnetic length) of 0.120 and stator inside diameter of 0.196 inches, the magnetic air-gap is 0.196−0.120=0.076 inches. Therefore the PC is approximately 0.120/0.076=1.6.
By way of example and with reference to FIG. 3, B/H curve of a typical high performance Neodymium Iron Boron Magnet is illustrated. The X axis represents coercivity (H) of the magnet. The Y axis represents the flux density (B). The numbers around the outside (starting at 0.1 and ending at 5.0 on this plot) are PC values, which dictate the operating point of the magnet. This plot illustrates that at a PC of 1.6 (as is the case for a typical actuator used in the current state of the art), the magnet operates at a flux density of 8.7 kilogauss when the temperature is 20 degrees C.
If the inside diameter of the stator tube is increased to 0.24 inches, by way of example, this will provide more than double the area for coil wires, easily allowing more than 22 turns of AWG #29 magnet wire to be used. However, increasing the inside diameter of the stator tube also increases the magnetic air-gap that the magnetic flux must jump across. Because of this, the magnetic field becomes weaker. This is shown in the plot of FIG. 4, indicated by the PC of 1.0. The weaker magnetic field requires even more coil turns to produce the same torque constant. The lower PC also creates a risk of demagnetization at elevated temperatures.
Analysis and testing have shown that the KM of a toothless actuator remains roughly the same between a PC of 1.0 and 2.0, and thus, there is essentially no well-known way to overcome the problem of heat generation within a toothless actuator. Therefore, if heat generation is a performance limiting factor, another type of actuator must be sought.
In the past, some companies have tried to overcome the problem of heat generation by using toothed actuators, also referred to as slotted actuators. By way of example, FIG. 5 illustrates a cross sectional view of one such actuator used in known optical scanners. In a toothed actuator, the coil is not located between the magnet and the stator steel, and instead is wound around a steel core which forms teeth around the magnet. Since the coil is no longer located between the magnet and the stator steel, the stator teeth can be much closer to the magnet. As a result, the PC of toothed actuators is much higher than for toothless actuators.
FIG. 6 illustrates the same magnet B/H curves as illustrated in FIGS. 3 and 4, but also highlights resulting flux density when the PC is 6. Since the magnet is operating at a higher flux density, only 38 turns of wire is required to generate 38,000 Dyne*Centimeters per amp, given the same rotor magnet described above. And since the coil area is much greater, thicker wire can be used.
Clearly a toothed stator arrangement can solve the heat generation problem. However, a new problem emerges which is one of greatly increased electrical inductance (L). For the actuator illustrated with reference to FIG. 5, by way of example, the inductance is greater than 300 microhenries, which is about three times the inductance of a toothless actuator with the same torque constant.
Referring again to FIG. 6 and now to FIG. 7, inductance is increased because of two factors. The first factor is external fringe lines which circulate magnetic flux around the coil, but do not interact with the rotor magnet to create torque. A second factor is tooth-to-tooth fringe lines which circulate magnetic flux around a gap between teeth and do not create torque.
To eliminate external fringe lines, the toothed stator could be rearranged, as illustrated with reference to FIG. 8, wherein the coils are wound around the teeth that are located completely contained inside the stator, essentially forming a series magnetic circuit between the two coils. Indeed this does help to reduce inductance to about 212 microhenries, but still undesirably more than double that of a toothless actuator producing the same torque.
To reduce the inductance even further, the tooth-to-tooth fringe must be reduced, and thus the gap between stator teeth must be opened up. For example, if the gap between stator teeth is increased to 0.050 inches, the inductance becomes 180 microhenries. If the gap between stator teeth is increased even further—to 0.070 inches, the inductance becomes 157 microhenries. This is still more than 50% higher than a slotless actuator, but may be tolerable for certain applications.
However, increasing the gap between stator teeth has negative consequences. The largest being that the actuator will tend to cog toward angles away from the center, since the North and South poles of the rotor magnet will strongly orient themselves in the direction of the stator teeth themselves. A small amount of cogging can be tolerated by a servo system located outside the optical scanner, but a large amount of cogging is detrimental to performance and thus, highly undesirable.
For example, with the toothed or slotted actuator described above with reference to FIG. 8, whose gap between teeth is 0.030 inches, the cogging torque is 14,000 Dyne*Centimeters at 20 degrees. When the gap between teeth is increased to 0.036 inches, the cogging torque is 22,000 Dyne*Centimeters at 20 degrees. When the gap between teeth is increased to 0.050 inches, the cogging torque increases to 40,000 Dyne*Centimeters at 20 degrees. When the gap between teeth is increased to 0.070 inches, the cogging torque increases to 85,000 Dyne*Centimeters at 20 degrees. A cogging torque of 14,000 Dyne*Centimeters is tolerable, but higher cogging torques are generally not tolerable.
Since limiting the inductance in a toothed actuator also means increasing the cogging torque, it would be expected that a toothed actuator should therefore not be used if inductance is a performance-limiting factor.
Further, as with electric motors and actuators that have teeth, normally each tooth, some made from laminations, is generally solid and each coil is typically wound on a fully assembled stator. As is appreciated in the art, winding a coil on such a stator is difficult and expensive, since the wire must first exist externally, and must be placed on each tooth turn-by-turn. This is difficult because of the close proximity between actuator teeth. In addition, it is also difficult to achieve optimal coil wire (typically copper) packing using such an approach. Such typical coil winding approaches are known to be expensive, and often result in a sub-optimal performance for the motor or actuator.
By way of example, one approach to improving coil packing is presented in US Patent Application Publication US 2013/0076185 for an Electromechanical limited Rotation Rotary Actuator by William R. Benner, Jr. For this Benner publication, the stator is segmented into multiple segments. In one embodiment, the actuator includes two stator sections, each fabricated from a plurality of laminated layers. The stator structure has discrete laminations employing a point-and-socket approach that allows the stator to be assembled as stator sections. Because of this, the coils extending around each tooth can be placed on each stator section very easily, since there is no other tooth to get in the way. Further, the coils can be wound directly onto a stator section by machine or alternatively, the coils can be separately wound onto a bobbin, or formed using bondable magnet wire, and then simply slid onto each of the teeth of each stator section. Once the coils are in place, the stator sections can be slid together. This construction provides a very inexpensive and easy way to assemble the stator assembly, and also allows for maximum conductor packing and thus a desirable actuator performance. While there are benefits to the use of segmented stators, there are drawbacks. By way of example, a segmented stator typically requires many more laminations that must be stacked. In the case of the segmented actuator as earlier referenced, there are two stacks of laminations with each stack having forty four separate pieces of metal layers, for a total of eighty eight laminations. If the stator were not segmented, there would only be forty four layers of metal forming the laminated stator. By way of further example for a stator having three teeth, there would need to be three times the number of laminations. It means that a human or a machine must stack many more layers to assemble the stator.
It is well known that motor and coil manufacturers are typically trying to obtain the highest flux, maximum lines of magnetic force, for a given amount of stator metal and magnetism. The more flux created, the more torque becomes available.
To reiterate, the typical toothless actuator is generally not capable of delivering a high torque constant along with a low coil resistance, and a typical toothed actuator is generally not capable of delivering low coil inductance. Further, typical actuators are generally more expensive to build, either because of increased costs of placing turns of wire on the teeth, or because of the increased cost of additional laminations needed in a segmented stator. Thus, there is clearly a need for an electromechanical rotary actuator that provides a high torque constant and a low coil resistance along with a low coil inductance. Further, there remains a need for a stator having a highly packed coil density for providing an efficiently operating actuator. Yet further, there is a need to provide such high density packing in an economical manner.